To me it's the fact that the game exists of zones rather than one huge open world. Everything feels instanced. If you enter a city you get a loading screen and you are isolated from the world. Every zone in the world is isolated in fact. Even inside the cities there are different zones with a loading screen that, for me at least, take away from the immersion.

That having been said, I still enjoy the game and will continue to play it for quite a while. But having the isolated zones replaced with open, larger zones would make me even happier.